Open up a new canvas, I will be using 350x125 and fill with a:
-Black Background for a darker sig or
-White Background for a lighter sig.
I used black.

Step 2

Next scale your render to fit inside the sig and paste it in on a new layer. I like to paste my render in early because it allows me to create my sigs around it, not making a background, pasting in the render then doing the foreground. This method saves a lot of unneeded trouble with blending in your render and overall flow of the sig.

Step 3

We are going to start adding some life to the background. Make a new transparent layer under your render layer but above the background. Call it 'BG Texture 1'. Whip out some of the grunge brushes from Brush Set 1, these should be called something like KD-Ultimate-ED-02. Set your foreground color to the major color in your render and make a couple of clicks on the layer to add some texture. Set this layer to Normal at 100%

Step 4

Make another transparent layer and call it 'BG Texture 2' and place it above everything except the render layer.. Now change your foreground color to black, and use a similar brush, if not the same brush to what you used in the previous step. Make another couple of clicks around the sig but remember to leave some room for the 1st texture layer to show through. Set this layer to normal in the Layers dialogue at 70%.

Step 5

Make another transparent layer above everything except the render layer. Call this layer whatever you want, just as long as you know what it is. Change your foreground color to white and make one click with a similar brush as the one used in the previous step somewhere near the focal point. Set this layer to Overlay at 100%.

Step 6

Make a new transparent layer and move it to the top of the pile (Above the render layer) and call it 'Clouds'. Set your foreground color to white and select one of the Sentinel Cloud Brushes (Brush Set 2). Make a couple of clicks near the top of the sig to represent clouds, but don't crowd it. Make sure to make some of the brush to go over the render, but not too much because your still want to see it clearly, just want to blend it into the sig a bit. Set this layer to Normal at 100%.

Step 7

Now go Edit>Copy Visible, Edit>Paste. Move this layer underneath the render layer and call it 'Solid BG 1'. I usually use this technique a couple of times in each sig, it helps build a string background which I can use brushes on easily and make them look good. Set this layer to Normal at 100%.

Step 8

Make a new transparent layer and move it above to 'Solid BG 1' layer and underneath the render layer, call this 'II BG Texture 1'. Set your FG color to the same as you used in Step 3, and make a couple of clicks around the sig. Next go Filters>Blur>Gaussian Blur and set both X and Y to 5. Set this Layer to Normal At 100%.

Step 9

Now we are going to start work on the foreground of the sig. Make a new transparent layer above the Render Layer but below the Clouds layer, call it 'FG Texture 1'. Get out an Ad-Hero brush (Brush Set 5), set your FG color to black and make a couple of clicks. Then change your FG color to white and make another couple of clicks, then switch back to black once more and make another couple. Set this layer to Overlay at 100%.

Step 10

Make another transparent layer and move it below the Cloud layer but above FG Texture 1, call it 'FG Texture 2'. Make your FG color black and select a fractal brush (Brush Set 3). Make a couple of clicks around the entire sig. Set this layer to Overlay at 100%.

Step 11

Make a new transparent layer and move it above the previous layer but below the Cloud Layer, call it 'FG Texture 3'. Make your FG color white, select a different fractal brush and make a couple of clicks around the sig. Set this layer to Overlay at 100% as well.

Step 12

Make a transparent layer and move it to the top of the pile (Above the Cloud layer), call it 'FG Texture 4'. Set your FG color to white and make one or two clicks at select parts of the sig where you want to add more texture, I only used one at the top right of the sig to blend in the clouds. Set this layer to Overlay at 100%.

Step 13

Now un-scale your render, but if it was a small render increase it's size. Now paste it in on a new layer, make the layer above the Fg Texture 4 layer and move this un-scaled render slightly to the side of the main render. Set this layer to Overlay at 80%.

Step 14

At the moment the colours of your sig are looking very plain, now it it time to fix that up. Go Edit>Copy Visible, Edit>Paste. Move this layer to the top if the pile and call it 'Color'. There are two main methods of colorizing a sig and I will explain them both, but in your sig only do one method.Colorize - Go Colors>Colorize (I am using Gimp 2.4.5 so this where it is found, not sure though about past versions). Experiment with the sliders until you get a color that matches your render reasonably well, then Lighten it up to about 20 on the slider and click 'OK'. Set this layer to Overlay at 100%. Now select your Render Layer, go Layer>Transparency>Alpha To Selection. Now go back to your Color layer and go Edit>Clear.Gradient Map - Set your FG and BG colors to the two main colors of your render. Now go Colors>Map>Gradient Map. Set this layer to Overlay at 100%.
Gradient Mapping is used for adding a more subtle amount of color whereas Colorizing is used to add a significant amount. I used the Colorize method.

Step 15

This step is optional, only do if your sig lacks contrast. Go Edit>Copy Visible, Edit>Paste. Move this to the top if the pile and set the layer to Overlay at 40%.

Finishing Touches

Now add some text and a border and your done. I like to do my borders by making a new transparent layer at the top of the pile, go Select>All, make your FG color something that suits the sig, like black for a dark sig. Then go Edit>Stroke Selection and set it to 2pxl.